LVM Building Blocks

Logical Volume Management makes use of the device-mapper feature of the Linux kernel to provide a system of partitions independent of the underlying disk's layout. With LVM you abstract your storage and have "virtual partitions", making it easier to extend and shrink partitions (subject to potential limitations of your file system) and add/remove partitions without worrying about whether you have enough contiguous space on a particular disk, getting caught up in fdisking a disk in use (and wondering whether the kernel is using the old or new partition table), or, having to move other partitions out of the way. This is strictly an ease-of-management issue: it does not provide any security. However, it sits nicely with the other two technologies we are using.

The basic building blocks of LVM are:

Physical volume (PV): Partition on hard disk (or even hard disk itself or loopback file) on which you can have volume groups. It has a special header and is divided into physical extents. Think of physical volumes as big building blocks which can be used to build your hard drive.

Volume group (VG): Group of physical volumes that are used as storage volume (as one disk). They contain logical volumes. Think of volume groups as hard drives.

Logical volume (LV): A "virtual/logical partition" that resides in a volume group and is composed of physical extents. Think of logical volumes as normal partitions.

Physical extent (PE): A small part of a disk (usually 4MiB) that can be assigned to a logical Volume. Think of physical extents as parts of disks that can be allocated to any partition.

Advantages

LVM gives you more flexibility than just using normal hard drive partitions:

Use any number of disks as one big disk.

Have logical volumes stretched over several disks.

Create small logical volumes and resize them "dynamically" as they get more filled.

Resize logical volumes regardless of their order on disk. It does not depend on the position of the LV within VG, there is no need to ensure surrounding available space.

Resize/create/delete logical and physical volumes online. File systems on them still need to be resized, but some support online resizing.

Online/live migration of LV being used by services to different disks without having to restart services.

Snapshots allow you to backup a frozen copy of the file system, while keeping service downtime to a minimum.

These can be very helpful in a server situation, desktop less so, but you must decide if the features are worth the abstraction.

Disadvantages

Linux exclusive (almost). There is no official support in most other OS (FreeBSD, Windows..).

Additional steps in setting up the system, more complicated.

If you use the Btrfs file system, its Subvolume feature will also give you the benefit of having a flexible layout. In that case, using the additional Abstraction layer of LVM may be unnecessary.

Installing Arch Linux on LVM

You should create your LVM Volumes between the Partitioning and formatting steps of the Installation Procedure. Instead of directly formatting a partition to be your root file system, it will be created inside a logical volume (LV).

Create partition(s) where your PV will reside. Set the partition type to 'Linux LVM', which is 8e if you use MBR, 8e00 for GPT.

Create your physical volumes (PV). If you have one disk it is best to just create one PV in one large partition. If you have multiple disks you can create partitions on each of them and create a PV on each partition.

When you reach the “Create initial ramdisk environment” step in the Beginners Guide, add the lvm hook to /etc/mkinitcpio.conf (see below for details).

Warning:/boot cannot reside in LVM when using GRUB Legacy, which does not support LVM. GRUB users do not have this limitation. If you need to use GRUB Legacy, you must create a separate /boot partition and format it directly.

Create physical volumes

Make sure you target the right partitions! To find the partitions with type 'Linux LVM':

MBR system: fdisk -l

GPT system: lsblk and then gdisk -l disk-device

Create a physical volume on them:

# pvcreate disk-device

disk-device may be e.g. /dev/sda2.
This command creates a header on each partition so it can be used for LVM.
You can track created physical volumes with:

# pvdisplay

Note: If using a SSD without partitioning it first, use pvcreate --dataalignment 1m /dev/sda2 (for erase block size < 1MiB), see e.g. here

Create volume group

Next step is to create a volume group on this physical volume. First you need to create a volume group on one of the new partitions and then add to it all other physical volumes you want to have in it:

# vgcreate VolGroup00 /dev/sda2
# vgextend VolGroup00 /dev/sdb1

Also you can use any other name you like instead of VolGroup00 for a volume group when creating it. You can track how your volume group grows with:

# vgdisplay

Note: You can create more than one volume group if you need to, but then you will not have all your storage presented as one disk.

Create logical volumes

Now we need to create logical volumes on this volume group. You create a logical volume with the next command by giving the name of a new logical volume, its size, and the volume group it will live on:

# lvcreate -L 10G VolGroup00 -n lvolhome

This will create a logical volume that you can access later with /dev/mapper/Volgroup00-lvolhome or /dev/VolGroup00/lvolhome. Same as with the volume groups, you can use any name you want for your logical volume when creating it.

To create swap on a logical volume, an additional argument is needed:

# lvcreate -C y -L 10G VolGroup00 -n lvolswap

The -C y is used to create a contiguous partition, which means that your swap space does not get partitioned over one or more disks nor over non-contiguous physical extents.

If you want to fill all the free space left on a volume group, use the next command:

# lvcreate -l +100%FREE VolGroup00 -n lvolmedia

You can track created logical volumes with:

# lvdisplay

Note: You may need to load the device-mapper kernel module (modprobe dm-mod) for the above commands to succeed:

Tip: You can start out with relatively small logical volumes and expand them later if needed. For simplicity, leave some free space in the volume group so there is room for expansion.

Create file systems and mount logical volumes

Your logical volumes should now be located in /dev/mapper/ and /dev/YourVolumeGroupName. If you cannot find them, use the next commands to bring up the module for creating device nodes and to make volume groups available:

# modprobe dm-mod
# vgscan
# vgchange -ay

Now you can create file systems on logical volumes and mount them as normal partitions (if you are installing Arch linux, refer to mounting the partitions for additional details):

Warning: When choosing mountpoints, just select your newly created logical volumes (use: /dev/mapper/Volgroup00-lvolhome). Do not select the actual partitions on which logical volumes were created (do not use: /dev/sda2).

Add lvm hook to mkinitcpio.conf

You will need to make sure the udev and lvm2mkinitcpio hooks are enabled.

udev is there by default. Edit the file and insert lvm2 between block and filesystem like so:

Configuration

Advanced options

If you need monitoring (needed for snapshots) you can enable lvmetad.
For this set use_lvmetad = 1 in /etc/lvm/lvm.conf.
This is the default by now.

You can restrict the volumes that are activated automatically by setting the auto_activation_volume_list in /etc/lvm/lvm.conf. If in doubt, leave this option commented out.

Grow physical volume

After changing the size of a physical volume (pv), e.g: growing a mdadm raid array, you need to grow the pv using the following command:

# pvresize /dev/mdX

Note: This command can be done while the volume is online

Grow logical volume

To grow a logical volume you first need to grow the logical volume and then the file system to use the newly created free space. Let us say we have a logical volume of 15 GB with ext3 on it, and we want to grow it to 20 GB. We need to do the following steps:

Warning: Not all file systems support growing without loss of data and/or growing online.

Note: If you do not resize your file system, you will still have a volume with the same size as before (volume will be bigger but partly unused).

Shrink logical volume

Because your file system is probably as big as the logical volume it resides on, you need to shrink the file system first and then shrink the logical volume. Depending on your file system, you may need to unmount it first. Let us say we have a logical volume of 15 GB with ext3 on it and we want to shrink it to 10 GB. We need to do the following steps:

Here we shrunk the file system more than needed so that when we shrunk the logical volume we did not accidentally cut off the end of the file system. After that, we normally grow the file system to fill all free space left on logical volume. You may use lvresize instead of lvreduce.

Do not reduce the file system size to less than the amount of space occupied by data or you risk data loss.

Not all file systems support shrinking without loss of data and/or shrinking online.

Note: It is better to reduce the file system to a smaller size than the logical volume, so that after resizing the logical volume, we do not accidentally cut off some data from the end of the file system.

Remove logical volume

Warning: Before you remove a logical volume, make sure to move all data that you want to keep somewhere else; otherwise, it will be lost!

First, find out the name of the logical volume you want to remove. You can get a list of all logical volumes installed on the system with:

# lvs

Next, look up the mountpoint for your chosen logical volume...:

$ df -h

... and unmount it:

# umount /your_mountpoint

Finally, remove the logical volume:

# lvremove /dev/yourVG/yourLV

Confirm by typing y and you are done.

Do not forget, to update /etc/fstab!

You can verify the removal of your logical volume by typing lvs as root again (see first step of this section).

Add physical volume to a volume group

You first create a new physical volume on the block device you wish to use, then extend your volume group

# pvcreate /dev/sdb1
# vgextend VolGroup00 /dev/sdb1

This of course will increase the total number of physical extents on your volume group, which can be allocated by logical volumes as you see fit.

Note: It is considered good form to have a partition table on your storage medium below LVM. Use the appropriate type code: 8e for MBR, and 8e00 for GPT partitions.

Remove partition from a volume group

All of the data on that partition needs to be moved to another partition. Fortunately, LVM makes this easy:

# pvmove /dev/sdb1

If you want to have the data on a specific physical volume, specify that as the second argument to pvmove:

# pvmove /dev/sdb1 /dev/sdf1

Then the physical volume needs to be removed from the volume group:

# vgreduce myVg /dev/sdb1

Or remove all empty physical volumes:

# vgreduce --all vg0

And lastly, if you want to use the partition for something else, and want to avoid LVM thinking that the partition is a physical volume:

# pvremove /dev/sdb1

Deactivate volume group

Just invoke

# vgchange -a n my_volume_group

This will deactivate the volume group and allow you to unmount the container it is stored in.

Snapshots

Introduction

LVM allows you to take a snapshot of your system in a much more efficient way than a traditional backup. It does this efficiently by using a COW (copy-on-write) policy. The initial snapshot you take simply contains hard-links to the inodes of your actual data. So long as your data remains unchanged, the snapshot merely contains its inode pointers and not the data itself. Whenever you modify a file or directory that the snapshot points to, LVM automatically clones the data, the old copy referenced by the snapshot, and the new copy referenced by your active system. Thus, you can snapshot a system with 35GB of data using just 2GB of free space so long as you modify less than 2GB (on both the original and snapshot).

Configuration

You create snapshot logical volumes just like normal ones.

# lvcreate --size 100M --snapshot --name snap01 /dev/mapper/vg0-pv

With that volume, you may modify less than 100M of data, before the snapshot volume fills up.

Reverting the modified 'pv' logical volume to the state when the 'snap01' snapshot was taken can be done with

# lvconvert --merge /dev/vg0/snap01

In case the origin logical volume is active, merging will occur on the next reboot.(Merging can be done even from a LiveCD)

The snapshot will no longer exist after merging.

Also multiple snapshots can be taken and each one can be merged with the origin logical volume at will.

The snapshot can be mounted and backed up with dd or tar. The size of the backup file done with dd will be the size of the files residing on the snapshot volume.
To restore just create a snapshot, mount it, and write or extract the backup to it. And then merge it with the origin.

It is important to have the dm_snapshot module listed in the MODULES variable of /etc/mkinitcpio.conf, otherwise the system will not boot. If you do this on an already installed system, make sure to rebuild the image with

snapshots are primarily used to provide a frozen copy of a file system to make backups; a backup taking two hours provides a more consistent image of the file system than directly backing up the partition.

If you have LVM volumes not activated via the initramfs, enable the lvm-monitoring service, which is provided by the lvm2 package.

Troubleshooting

Changes that could be required due to changes in the Arch-Linux defaults

The use_lvmetad = 1 must be set in /etc/lvm/lvm.conf. This is the default now - if you have a lvm.conf.pacnew file, you must merge this change.

LVM commands do not work

Load proper module:

# modprobe dm_mod

The dm_mod module should be automatically loaded. In case it does not, you can try:

/etc/mkinitcpio.conf:

MODULES="dm_mod ..."

You will need to rebuild the initramfs to commit any changes you made.

Try preceding commands with lvm like this:

# lvm pvdisplay

Logical Volumes do not show up

If you are trying to mount existing logical volumes, but they do not show up in lvscan, you can use the following commands to activate them:

# vgscan
# vgchange -ay

LVM on removable media

Symptoms:

# vgscan
Reading all physical volumes. This may take a while...
/dev/backupdrive1/backup: read failed after 0 of 4096 at 319836585984: Input/output error
/dev/backupdrive1/backup: read failed after 0 of 4096 at 319836643328: Input/output error
/dev/backupdrive1/backup: read failed after 0 of 4096 at 0: Input/output error
/dev/backupdrive1/backup: read failed after 0 of 4096 at 4096: Input/output error
Found volume group "backupdrive1" using metadata type lvm2
Found volume group "networkdrive" using metadata type lvm2

Cause:

Removing an external LVM drive without deactivating the volume group(s) first. Before you disconnect, make sure to:

# vgchange -an volume group name

Fix: (assuming you already tried to activate the volume group with # vgchange -ay vg, and are receiving the Input/output errors:

# vgchange -an volume group name

Unplug the external drive and wait a few minutes:

# vgscan
# vgchange -ay volume group name

Kernel options

In kernel options, you may need dolvm. root= should be set to the logical volume, e.g /dev/mapper/vg-name-lv-name.